Several publications describe the preparation of cyclohexadienic derivatives of the type of those of formula (III). For instance, the preparation of 2,6,6-trimethyl-cyclohexa-1,3-dien-1-yl methyl ketone [X=CH.sub.3 in formula (III)] has been disclosed in Chem. Comm. 1973, 161. 2,6,6-Trimethyl-cyclohexa-1,3-dien-1-carboxylic acid, as well as the corresponding methyl and ethyl esters [X=OH, OCH.sub.3 or OC.sub.2 H.sub.5 in formula (III)], are prepared from the corresponding mono-unsaturated compound of formula (IV) ##STR7## (X=OH, OCH.sub.3, OC.sub.2 H.sub.5), by reacting them with a halogen and subsequently dehydrohalogenating the obtained halo-derivative [see e.g. Compt. Rend. Ser. C, 262, 1725 (1966) and Helv. Chim. Acta 38, 1863 (1955)]. These methods however have not been developed for industrial scale preparations as they have been found uneconomical and difficult to scale-up. These disadvantages of the prior known syntheses have been overcome by the process of the present invention.